"Clearly we are in doubt as to
the proper criterion of life.
D'Herelle says that the bacteriophage is alive, because, like the
flea or the tiger, it can multiply
indefinitely at the cost of living
beings. His opponents say that it
can multiply only as long as
its food is alive, whereas the
tiger certainly, and the flea
probably, can live on dead products of life. They suggest that
the bacteriophage is like a book
or a work of art, which is
constantly being copied by living beings, and is therefore only
metaphorically alive, its real life being in its copiers."